1. Field of the Invention
The present invention relates to a method for detecting wires bonded between the pads of semiconductor chips and the leads of lead frames.
2. Prior Art
The method described in U.S. Pat. No. 5,347,362, which corresponds to Japanese Patent Application Laid-Open (Kokai) No. 5-160233, is an example of one of the conventional methods for detecting bonded wires, particularly the height of the bonded wires. FIG. 6 shows the detection apparatus used in this method.
An exemplary workpiece 1 that is to be detected by this apparatus includes, as shown in FIG. 7, a semiconductor chip 5 installed on a lead frame 3 by a paste 4 which is an epoxy resin, etc., and wires 7 are bonded so as to connect the pads 6 of the semiconductor chip 5 to the leads 2 of the lead frame 3.
The detection apparatus of FIG. 6 includes an illuminating means 11 located above a detection stand 10 on which the workpiece 1 is placed for inspection. The illuminating means 11 is mounted to the lower part of an optical system 12, and an imaging device 14 such as a CCD (photoelectric transducer element) camera, etc. is mounted on the upper part of the optical system 12 with a diaphragm means 13 in between. The optical system 12 to which the illuminating means 11 and imaging device 14 are mounted is attached in a vertically movable fashion to a supporting block 16 which is installed on the surface of an XY table 15. The optical system 12 is moved up and down by a Z-axis motor 17.
As shown in FIGS. 8 and 9, the illuminating means 11 includes a high-angle illuminating device 22 and a low-angle illuminating device 24. In the illuminating device 22, LED's 21 are installed in the form of a ring around an opening 20a that is formed in an illumination retaining plate 20; and in the illuminating device 24, LED's 24 are also installed in the form of a ring so as to be positioned on the outside of the illuminating device 22. The LED's 21 and 24 of the high-angle illuminating device 22 and low-angle illuminating device 24 are oriented toward the optical axis of the optical system 12. The angle of inclination of the high-angle illuminating device 22 relative to a horizontal plane is set to be approximately 30 to 55 degrees, and the angle of inclination of the low-angle illuminating device 24 relative to the horizontal plane is set to be approximately 5 to 15 degrees.
When the height of a bonded wire is to be detected, the low-angle illuminating device 24 of the illuminating means 11 is controlled so that a part of its illumination is turned off as disclosed in the above-identified prior art, and the focal depth of the optical system 12 is set to be shallow via the diaphragm means 13.
More specifically, when wire detection is performed, the XY table 15 is moved horizontally so that the optical system 12 is, as shown in FIG. 7(a), positioned on a vertical line A which passes through the XY coordinates of one point on the wire 7 that is to be detected. Then, the Z-axis motor 17 is activated. The optical system 12 is raised and lowered by the motor 17 and focused at the detection point A0, resulting in that the image of the wire 7 appears as shown in FIG. 7(b). As seen from FIG. 7(b), the wire width H0 at the detection point A0 is imaged so as to appear with a minimum value, and the width of the image wire becomes larger and less distinct as the distance from the focal point A0 increases.
As disclosed in the prior art, there is a particular relationship between the blurring width (wire width) and the wire height Z as disclosed in the above-identified prior art; accordingly, if the relationship between the width and height beforehand were determined beforehand, the height of the bonded wire at an arbitrary point can easily be calculated (measured) from the wire width.
In the prior art described above, the wire height is measured based upon the imaged wire width using the in-focus wire image as a standard. In other words, the wire 7 is detected on the basis of the contrast between the wire 7 and the background. However, on a lead frame, the paste is squeezed out around the periphery of the semiconductor chip; and these squeezed out paste is imaged as bright areas by the imaging device. As a result, regions which are in the background of the wire and in which such squeezed-out paste is present cannot be contrasted with nor distinguished from the wire. In other words, a portion of the end part of the wire are not detected clearly.